SQL where子句中联合的替代方法

Question

in where clause i am using like

SELECT name 
  FROM products 
 WHERE id IN (
           SELECT product_id 
             FROM orders 
            UNION 
           SELECT 1);

it taking time to perform union .

please provide some other alternative to improve my query performance

Answer 1

what about

select name from products
where id in (select product_id from orders)
    or id = 1

Answer 2

A union removes duplicates, and SQL Server may unfortunately not be able to spot that such removal is not required when the result of the union is being used in an IN predicate.

SELECT name 
  FROM products 
 WHERE id IN (
           SELECT product_id 
             FROM orders 
            UNION ALL
           SELECT 1);

UNION ALL says that duplicates are allowed, so it will avoid the expensive step of removing duplicates. Even though you may think that, because the second part of the union only has one value, the duplicate check should be quick, this isn't so. UNION says that all duplicates must be eliminated. It has to eliminate all the duplicate product_ids from the first part of the query also.

---

I've just done a few quick tests, and found one repro where the optimizer isn't smart enough to avoid duplicate removal, for versions 2000, 2005, 2008. All 3 show a query plan that shows a distinct sort being used after concatenation between a table scan (of #IDs ) and a constant scan:

create table #IDs (
    ID int not null
)
go
insert into #IDs (ID)
select 1 union all
select 1 union all
select 2 union all
select 2 union all
select 3
go
select * from sysobjects where id in (select ID from #IDs union select 1)
go

Answer 3

Use the following query. Most efficient of the lot.

select name from products P
where P.id = 1 or 
exists 
(select TOP 1 '1' from orders O
where O.product_id = P.id)

Answer 4

select p1.name from products as p1
    inner join orders as o on o.product_id = p1.id
union
select p2.name from products as p2 where p2.id = 1;

Answer 5

减少总的累积CPU时间：9秒900毫秒结束的作业= job_1442292787903_0035启动的MapReduce作业：Stag